1. Field of the Invention
The present invention relates to an engine control unit for driving an electric circuit, particularly a starter in a vehicle.
2. Description of the Related Art
Engine control units are employed in automotive engineering and are used there for controlling engine components, e.g., a starter of the engine or a fuel pump. Via inputs and outputs, corresponding engine components are able to be connected to the engine control unit in order to control them. For example, the battery voltage is applied via a first input of the engine control unit, and via a second input, a signal may be made available, e.g., an ignition signal, which then actuates corresponding switches within the engine control unit, so that ultimately the starter or the fuel pump is able to be energized, and the engine is then started by the switching-on of the starter and the fuel pump.
The switches are usually either connected on one side to the positive pole of the battery voltage, and after a suitable signal of the microcontroller, switch a starter relay, etc., to ground. Such switches are known as high-side switches. However, there is also the opposite case, in which what is termed a low-side switch connects the starter relay or the fuel-pump relay to the negative pole of the battery voltage.
To allow detection of faults in the electric circuits during operation of a vehicle, e.g., short circuits of the starter to negative or ground, published German patent document DE 196 165 43 C1 or published German patent application document DE 44 02 115 A1 have described suitable devices having special circuitry or engine control units that permit fault detection of short circuits.
Moreover, in order to increase safety with respect to faults, especially with respect to short circuits when starting vehicles, it has become known to provide both a high-side and a low-side switch. The operating principle is as follows: In the engine control unit, a high-side switch is provided that is connected on one side to the battery voltage, and on the other side, to a first output of the engine control unit. The high-side switch is driven via a microcontroller of the engine control unit. Also provided in the engine control unit is a low-side switch which is connected to an output of the engine control unit—the output being connected to ground—and to a second input. Connected between the first output and the second input of the engine control unit is a relay for the starter, which energizes the electric circuit of the starter only when both the high-side and the low-side switch are switched on, that is, are conductive. Therefore, in response to a short circuit of an individual switch, the electric circuit of the starter is not supplied with current, thereby increasing safety, since it is undesirable for a starter or a fuel pump to start up in uncontrolled manner.
However, there is a trend to minimize the weight of cables, etc., in order to keep the overall weight of the vehicle as low as possible. Therefore, in practice, either the high-side switch or the low-side switch is often short-circuited. This saves on weight, since it is possible to dispense with additional cable for the connection of the relay to the second input of the engine control unit. In the case of the short circuit of the high-side switch, the relay of the starter is thus connected directly to the battery voltage; in the case of the short circuit of the low-side switch, the relay of the starter is connected directly to ground. It may be that there are thus savings in cable and weight; however, the likelihood that the starter will switch on in uncontrolled fashion is consequently increased, since in each case, a relay only opens or closes the electric circuit of the starter with one switch.